This invention relates to electrostatic printing devices and more particularly to non-impact printing devices which utilize electronically addressable printheads for depositing developer in image configuration on plain paper substrates.
Of the various electrostatic printing techniques, the most familiar and widely utilized is that of xerography wherein latent electrostatic images formed on a charge retentive surface are developed by a suitable toner material to render the images visible, the images being subsequently transferred to plain paper.
A lesser known form of electrostatic printing is one that has come to be known as Direct Electrostatic Printing (DEP). This form of printing differs from the aforementioned xerographic form, in that, the toner or developing material is deposited directly onto a plain (i.e. not specially treated) substrate in image configuration. This type of printing device is disclosed in U.S. Pat. No. 3,689,935 issued Sept. 5, 1972 to Gerald L. Pressman et al. In general, this type of printing device uses electrostatic fields associated with addressable electrodes for allowing passage of developer material through selected apertures in a printhead structure. Additionally, electrostatic fields are used for attracting developer material to an imaging substrate in image configuration.
Pressman et al disclose an electrostatic line printer incorporating a multilayered particle modulator or printhead comprising a layer of insulating material, a continuous layer of conducting material on one side of the insulating layer and a segmented layer of conducting material on the other side of the insulating layer. At least one row of apertures is formed through the multilayered particle modulator. Each segment of the segmented layer of the conductive material is formed around a portion of an aperture and is insulatively isolated from every other segment of the segmented conductive layer. Selected potentials are applied to each of the segments of the segmented conductive layer while a fixed potential is applied to the continuous conductive layer. An overall applied field projects charged particles through the row of apertures of the particle modulator and the density of the particle stream is modulated according to the pattern of potentials applied to the segments of the segmented conductive layer. The modulated stream of charged particles impinge upon a print-receiving medium interposed in the modulated particle stream and translated relative to the particle modulator to provide line-by-line scan printing. In the Pressman et al device the supply of the toner to the control member is not uniformly effected and irregularities are liable to occur in the image on the image receiving member. High-speed recording is difficult and moreover, the openings in the printhead are liable to be clogged by the toner.
U.S. Pat. No. 4,491,855 issued on Jan. 1, 1985 in the name of Fuji et al discloses a method and apparatus utilizing a controller having a plurality of openings or slit-like openings to control the passage of charged particles and to record a visible image of charged particles directly on an image receiving member. Specifically, disclosed therein is an improved device for supplying the charged particles to a control electrode that has allegedly made high-speed and stable recording possible. The improvement in Fuji et al lies in that the charged particles are supported on a supporting member and an alternating electric field is applied between the supporting member and the control electrode. Fuji et al purports to obviate at least some of the problems noted above with respect to Pressman et al. Thus, Fuji et al alleges that their device makes it possible to sufficiently supply the charged particles to the control electrode without scattering them.
U.S. Pat. No. 4,568,955 issued on Feb. 4, 1986 to Hosoya et al discloses a recording apparatus wherein a visible image based on image information is formed on an ordinary sheet by a developer. The recording apparatus comprises a developing roller spaced at a predetermined distance from and facing the ordinary sheet and carrying the developer thereon. It further comprises a plurality of addressable recording electrodes and corresponding signal sources connected thereto for attracting the developer on the developing roller to the ordinary sheet by generating an electric field between the ordinary sheet and the developing roller according to the image information. A plurality of mutually insulated electrodes are provided on the developing roller and extend therefrom in one direction. A.C. and D.C. voltage sources are connected to the electrodes, for generating alternating electric fringe fields between adjacent ones of the electrodes to cause oscillations of the developer positioned between the adjacent electrodes along electric lines of force therebetween to thereby liberate the developer from the developing roller.
Direct electrostatic printing (DEP) structures are particularly attractive due to reduced manufacturing cost and increased reliability opportunities in non-impact electronic printing. DEP printing systems which utilize apertured printhead structures such as those of Pressman et al and Fuji et al have the potential problem of reduced performance due to aperture clogging. Aperture clogging is caused by wrong sign toner accumulating on the control electrode structure of the apertured printhead structure. A typical printhead structure comprises a shield electrode structure and a control electrode structure which are supported on opposite sides of an insulating member. The printhead structure together with a suitable supply of toner particles and appropriate electrical bias voltages are usually arranged such that the shield electrode structure faces the toner supply.
The problem of aperture clogging through accumulation of wrong sign toner particles on the control electrode structure is addressed in a number of patents. Generally, the problem is solved by minimizing the amount of wrong sign toner in the toner supply or by the provision of structure for cleaning or removing toner from the control electrode structure.
U.S. Pat. No. 4,743,926 granted to Schmidlin et al on May 10, 1988 and assigned to the same assignee as the instant invention discloses an electrostatic printing apparatus including structure for delivering developer or toner particles to a printhead forming an integral part of the printing device. Alternatively, the toner particles can be delivered to a charge retentive surface containing latent images. The developer or toner delivery system is adapted to deliver toner containing a minimum quantity of wrong sign and size toner. To this end, the developer delivery system includes a pair of charged toner conveyors which are supported in face-to-face relation. A bias voltage is applied across the two conveyors to cause toner of one charge polarity to be attracted to one of the conveyors while toner of the opposite is attracted to the other conveyor. One of charged toner conveyors delivers toner of the desired polarity to an apertured printhead where the toner is attracted to various apertures thereof from the conveyor.
In another embodiment of the '926 patent a single charged toner conveyor is supplied by a pair of three-phase generators which are biased by a DC source which causes toner of one polarity to travel in one direction on the electrode array while toner of the opposite polarity travels generally in the opposite direction.
In an additional embodiment disclosed in the '926 patent, a toner charging device is provided which charges uncharged toner particles to a level sufficient for movement by one or the other of the aforementioned charged toner conveyors.
U.S. Pat. No. 4,814,796 granted to Fred W. Schmidlin on Mar. 3, 1989 and assigned to the same assignee as the instant invention discloses a direct electrostatic printing apparatus including structure for delivering developer or toner particles to a printhead forming an integral part of the printing device. The printing device includes, in addition to the printhead, a conductive shoe which is suitably biased during a printing cycle to assist in the electrostatic attraction of developer through apertures in the printhead onto the copying medium disposed intermediate the printhead and the conductive shoe. The structure for delivering developer or toner is adapted to deliver toner containing a minimum quantity of wrong sign toner. To this end, the developer delivery system includes a conventional magnetic brush which delivers toner to a donor roll structure which, in turn, delivers toner to the vicinity of apertures in the printhead structure.
U.S. Pat. No. 4,755,837 granted to Fred W. Schmidlin on July 5, 1988 and assigned to the same assignee as the instant invention discloses a direct electrostatic printing apparatus including structure for removing wrong sign developer particles from a printhead forming an integral part of the printing device. The printing device includes, in addition to the printhead, a conductive shoe which is suitably biased during a printing cycle to assist in the electrostatic attraction of developer passing through apertures in the printhead onto the copying medium disposed intermediate the printhead and the conductive shoe. During a cleaning cycle, the printing bias is removed from the shoe and an electrical bias suitable for creating an oscillating electrostatic field which effects removal of toner from the printhead is applied to the shoe.
U.S. Pat. No. 4,912,489 discloses a Direct Electrostatic Printing device comprising a printhead structure comprising a shield electrode structure and a control electrode structure supported by an insulative support member. The printhead structure is positioned such that the control electrode is opposite the toner supply. Wrong sign toner accumulates on the control electrode.